This invention relates to a process for preparing a flame retardant product predominant in decabromodiphenylalkane and containing less than 100 ppm excess bromine.
Polybromodiphenylalkanes, e.g. decabromodiphenylethane, are known flame retardants for use in polyolefin and polystyrenic-based formulations. On a commercial basis, the polybromodiphenylalkane would be supplied to the formulation as a product predominant in the polybromodiphenylalkane selected. The product would have a form and an impurity content which would be characteristic of the process used to produce it. If the product's physical characteristics, e.g. thermal stability, limit the formulation's processability, then the processor's desire for the product is limited at best. If the product's color is not white or at least near white, the product will be suitable for use in some formulations, however, the product's use may not be acceptable in formulations calling for a white or light color. One impurity which may affect the product's color is excess bromine.
By excess bromine is meant bromine which is in excess of the 10 bromine atoms which are attached to the carbon atoms of the aromatic rings. It is believed that at least a portion of the excess bromine is elemental bromine which may be trapped in the particle structure. Before utilizing the product in flame retardant formulations, it is highly desirable to remove as much of the excess bromine from the product as possible. Removal of excess bromine not only improves product color it also assures that formulations containing the polybromodiphenylalkane as a flame retardant will be less corrosive to the equipment used in preparing and processing the flame retardant formulations.
A well known method for removal of impurities from brominated aromatic compounds is taught by Ayres et al. U.S. Pat. No. 4,327,227. However, the removal of the excess bromine impurity from a polybrominated diphenylalkane predominant product is surprisingly much more difficult than the removal of impurities from the brominated aromatic compounds taught in Ayres et al. At highly elevated temperatures, it is believed that the excess bromine may tend to brominate the alkylene bridge linking the aromatic rings of a polybrominated diphenylalkane molecule. For example, when the diphenylalkane to be brominated is diphenylethane, an amount of dodecabromodiphenylethane may be formed at the temperature used to remove most of the excess bromine from the product.
A product containing a substantial amount of dodecabromodiphenylethane may or may not be desirable depending on the formulation in which it is used as a flame retardant. Generally, the higher the degree of bromination, the greater the degree of flame retardancy. However, for any given degree of bromination, formulations containing brominated aliphatic flame retardant products are somewhat less thermally and/or UV stable than formulations utilizing flame retardant products consisting essentially of brominated aromatic compounds.